In plastic containers adapted to contain food, carbonated beverages and the like, it is highly desirable that the container be formed of material providing wall structures of low gas permeability to allow the food and beverages to be stored over long periods of time without going stale or flat, respectively. It is often highly desirable that the container material be totally transparent so that the material stored in the container can be viewed by the consumer. In addition, it is necessary that the material forming the container or, at least the material of the inner surface of the container which is in contact with the food or beverage, have the approval of the United States Food and Drug Administration (FDA).
Polyethylene terephthalate (PET) is an FDA approved material which is widely used in forming plastic beverage containers. In addition, PET which has been recovered from previously used containers and the like, so-called recycled polyethylene terephthalate (RPET), is also used in making plastic beverage containers. While newly polymerized polyethylene terephthalate (commonly referred to as virgin PET) has, as noted above, FDA approval, RPET is not FDA approved and thus cannot be used in direct contact with the beverage or food. This has led to a practice in which beverage containers are formed of layered materials, with the inner layer of virgin PET being in contact with the beverage and the outer layer of RPET being on the outside of the container. Both these layers may be formed together by various techniques involving fusion or co-molding, with or without an adhesive layer between the PET layers.
Numerous methods have been disclosed in the art for forming PET containers, primarily used in making PET bottles for use with liquids or beverages. Some of these methods include the use of multiple layers of material with efforts to keep FDA approved layers on the inside in contact with the food or beverage with non-FDA approved layers outside. One of the most common features of these methods is the use of extrusion or injection molding to create a pre-form or blank container of smaller size than actually required which is then blow molded into the appropriate size. Examples of this type of practice would include U.S. Pat. No. 4,587,073 to Jakobsen and U.S. Pat. No. 5,085,821 to Nohara. In the '073 patent two layers are coextruded to form a blank. In this process, the patent discloses that it may be expedient to extrude continuously a tube which is thereupon cut into pieces of suitable length. These pieces of tubing are enclosed at one end while at the same time being shaped at their other end in order to permit their fastening in a forming apparatus. The '073 patent discloses that the closing process involves fastening the tube over a mandrel, heating the part of the blank to be closed and then closing the blank around the end of the mandrel to assume the desired rounded final shape. The '821 patent also discloses the use of coextrusion of a multilayer pipe, cutting it into a predetermined length, and then closing one end of the cut pipe by fusion bonding to form a bottom portion, and then forming the other end of the cut pipe into a neck portion having an opening in the top end and a fitted or screwed part on the periphery. In both the '073 and '821 patents, after the preformed blank is extruded or otherwise manufactured, the final product is reached by blow molding the preform into the desired shape and size.
While it would be significantly less expensive to be able to create PET based containers by simply extruding them without need for an injection molding or blow molding step, the industry has consistently maintained the use of pre-forms followed by blow molding. In large part, this is because of the difficulty in successfully extruding PET tubes or PET based tubes of appropriate diameters and thicknesses to provide the final structure for a can or other container.
The prior art does provide some base methods for extruding thermoplastic tubing. Examples include U.S. Pat. No. 5,630,982 to Boring and U.S. Pat. No. 5,085,567 to Neumann et al. The disclosures of these patents, which are incorporated herein by reference, provide information on the use of calibration systems for maintaining tubular shape and low eccentricity in the extrudate while it is in its cooling process. While these patents address extruding of thermoplastic materials, they do not specifically address the possibilities and problems of extruding PET in larger diameters and/or smaller thicknesses.
Plastic tubes, such as those suggested in the current invention (generically referred to herein as profiles, and more specifically as pipes or tubes), can be produced by an extrusion process in which dry polymeric raw materials are passed to an extruder which employs one or more screw-type devices which knead and compress the raw material. Heat is applied in the extruder and the combination of heat and pressure turn the dry raw material into a molten plastic. At the discharge end of the extruder, the molten plastic is forced through a die, more specifically between an outer die portion and a central die insert.
As the hot plastic tubing exists the die, it is passed into a vacuum calibrated box which is maintained at reduced pressure and filled with a cooling fluid, typically water. Within the vacuum calibration box is a sizing sleeve or collar, possibly in the form of a series of wafers, which is smaller in diameter than the tubing exiting the die. Because an axial force is applied to the hot tubing as it exits the die, the tubing is reduced in diameter and thickness before it enters the vacuum calibrated box, which is called "draw down."
The center of the extruded tubing is maintained at atmospheric pressure, while the exterior is subjected to reduced pressure in the vacuum calibration box. The pressure within the tubing thus tends to expand the tubing against the sizing collar and the result is tubing of a fairly uniform outer diameter. Another common feature in vacuum calibration systems is the use of a spray of water within the system itself against the outside surface of the extruded pipe as it is passed through the calibration chamber. This wet calibration has in practice established itself over dry calibration processes because the water may act like a lubricant between the extruded pipe and the inside of the wall of the calibrating sleeve within the calibration chamber. While the prior art addresses generally the tools, including calibration, for successfully extruding thermoplastic pipes in general, a significant gap exists in successfully calibrating PET based plastic pipes or tubes of the desired diameter and thickness.
Multiple layer containers present an additional consideration. PET is relatively permeable to carbon dioxide and oxygen so the containers formed of PET have a relatively short shelf life. In order the prolong the shelf life of such containers, it is known in the art to incorporate a barrier material in such containers. Typically, such containers may be formed of an interior layer of virgin PET (referred to herein as PET, as compared with RPET for recycled PET), a barrier layer, and an outer layer formed of RPET. Containers of this nature are disclosed in U.S. Pat. No. 5,464,016 to Slat et al. As disclosed in Slat, a suitable container configuration includes an inner layer formed of PET or polyethylene naphthylate, an outer layer of RPET, and an intermediate barrier layer which may be formed of acrylonitrile copolymers, ethylene vinyl alcohol copolymers, vinyladene chloride copolymers, and copolymers of vinyladene chloride with vinyl chloride or methylacrylate. Various procedures are disclosed in Slat for forming the containers of three layers, i.e., an interior layer of an FDA approved polymer, an intermediate barrier layer, and an outer layer such as RPET which does not have FDA approval. One technique involves the application of an inner layer polymer and a barrier layer polymer which are applied to an interior mold to make a preform. This can be accomplished by various techniques, including coextrusion. This is followed by various procedures which can then involve an injection molding technique in which the outer layer is applied over the preform. The preform is then subjected to a blow molding operation to arrive at the final product. Another technique for forming beverage containers and similar multilayered articles involves so-called lamellar injection molding such as disclosed in U.S. Pat. No. 5,202,074 to Schrenk et al., which is incorporated herein by reference. As disclosed in the Schrenk patent, a plurality of thermoplastic polymers can be applied through respective extruders to a coextrusion feedblock which functions to generate and arrange layers in any of a number of configurations. As described in Schrenk, using the designation of "A," "B," and "C" for three different polymers applied through extruders to a coextrusion feedblock system, layer orientations of A B C, A B A B A, or A B C B A configurations can be arrived at. In addition to the orientation of the various polymer materials, the thickness of individual layers can likewise be controlled, and in competitive multiplication of the lamellar injection technique, the several polymer materials can be extruded in such thin layers that they become centrally a homogenous material. The Schrenk process discloses the such lamellar injection systems in the production of plastic beverage containers involving multilayer structures involving an FDA approved material such as PET with a barrier material such as ethylene vinyl alcohol.
An alternative system, somewhat related to the lamellar injection system, which may be used to produce multilayer extruded pipes is the modular disk die discussed in the article "Back to Basics with Annular Coextrusion, the Invention of the Modular Disk Die", by Henry G. Schirmer. The modular disk die disclosed in these materials consists of four basic elements: a melt inlet plate; the melt dividing module; the mandrel assembly; and the lower exit plate. This annular coextrusion die uses modules of assembled disks which can be stamped from thinner metal or machined from thicker metal. These modules define the layers and structural arrangement. In one disclosed embodiment, the die is able to handle 12 separate melts or less and distribute them in discrete layers of any desired configuration. This MDD or modular disk die provides another alternative to the LIM (lamellar injection molding) discussed in the preceding paragraph.
Effective barrier materials used in the fabrication of container parisons are fusion blends of PET and polyester based copolymers as disclosed in U.S. Pat. No. 4,578,215 to Jabarin. As disclosed in Jabarin, such barrier materials include copolymers such as copolymers of terephthalic acid and isothalic acid with one or more diols, particularly ethylene glycol in combination with other dihydroxy alcohols, specifically, 1, 3 bis (2 hydroxy ethoxy) benzene. Other suitable reactants include cell foams such as disk (4-beta-hydroxy ethoxy phenol) cell foam and additives such as stabilizers, processing aids, pigments, etc. The barrier materials thus formulated can be mixed with PET to form intimate fusion blends of 80 to 90 percent PET and 10 to 20 percent polyester to form barriers that are about 20 to 40 percent gas barriers to CO.sub.2 transmission than PET alone.
Barrier materials of the type disclosed in the Jabarin reference have heretofore been used in formulations of long shelf life containers by using such materials as blends with another FDA approved material such as PET. As disclosed in a paper by Suematsu, "Growth Prospects and Challenges for PET in Asia/Japan Producers Perspective," presented in Singapore, May 19-20, 1997, a commercially available copolyester of the type disclosed in the Jabarin patent can be blended with PET to provide a material of substantially lower permeability of carbon dioxide and PET. This product identified as copolyester B010 is said to have substantially better barrier properties than polyethylene naphthylate and to be useful as a blend with PET to form a barrier material having FDA approval.